Is the ocean surface a source of nitrous acid (HONO) in the marine boundary layer?
<p>Nitrous acid, <span class="inline-formula">HONO</span>, is a key net photolytic precursor to <span class="inline-formula">OH</span> radicals in the atmospheric boundary layer. As <span class="inline-formula">OH</span> is the...
| Main Authors: | , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2021-12-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/21/18213/2021/acp-21-18213-2021.pdf |
| Summary: | <p>Nitrous acid, <span class="inline-formula">HONO</span>, is a key net photolytic precursor to <span class="inline-formula">OH</span>
radicals in the atmospheric boundary layer. As <span class="inline-formula">OH</span> is the dominant
atmospheric oxidant, driving the removal of many primary pollutants and the
formation of secondary species, a quantitative understanding of <span class="inline-formula">HONO</span> sources is important to predict atmospheric oxidising capacity. While a number of
<span class="inline-formula">HONO</span> formation mechanisms have been identified, recent work has ascribed
significant importance to the dark, ocean-surface-mediated conversion of
<span class="inline-formula">NO<sub>2</sub></span> to <span class="inline-formula">HONO</span> in the coastal marine boundary layer. In order to evaluate
the role of this mechanism, here we analyse measurements of <span class="inline-formula">HONO</span> and related
species obtained at two contrasting coastal locations – Cabo Verde
(Atlantic Ocean, denoted Cape Verde herein), representative of the clean remote tropical marine
boundary layer, and Weybourne (United Kingdom), representative of
semi-polluted northern European coastal waters. As expected, higher average
concentrations of <span class="inline-formula">HONO</span> (70 ppt) were observed in marine air for the more
anthropogenically influenced Weybourne location compared to Cape Verde (<span class="inline-formula">HONO</span> <span class="inline-formula"><i><</i></span> 5 ppt). At both sites, the approximately constant <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M14" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">HONO</mi><mspace linebreak="nobreak" width="0.125em"/><mo>/</mo><mspace width="0.125em" linebreak="nobreak"/><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="65pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="2b45f25831a74b6b87a56a771d67638f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-18213-2021-ie00001.svg" width="65pt" height="14pt" src="acp-21-18213-2021-ie00001.png"/></svg:svg></span></span>
ratio at night pointed to a low importance for the dark, ocean-surface-mediated conversion of <span class="inline-formula">NO<sub>2</sub></span> into <span class="inline-formula">HONO</span>, whereas the midday maximum in the
<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">HONO</mi><mspace width="0.125em" linebreak="nobreak"/><mo>/</mo><mspace width="0.125em" linebreak="nobreak"/><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="65pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="180349e123cb250f82d08dca8e4cb15f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-18213-2021-ie00002.svg" width="65pt" height="14pt" src="acp-21-18213-2021-ie00002.png"/></svg:svg></span></span> ratios indicated significant contributions from photo-enhanced
<span class="inline-formula">HONO</span> formation mechanisms (or other sources). We obtained an upper limit to
the rate coefficient of dark, ocean-surface <span class="inline-formula">HONO</span>-to-<span class="inline-formula">NO<sub>2</sub></span> conversion of
<span class="inline-formula"><i>C</i><sub>HONO</sub></span> <span class="inline-formula">=</span> 0.0011 ppb h<span class="inline-formula"><sup>−1</sup></span> from the Cape Verde observations; this
is a factor of 5 lower than the slowest rate reported previously. These
results point to significant geographical variation in the predominant <span class="inline-formula">HONO</span>
formation mechanisms in marine environments and indicate that caution is
required when extrapolating the importance of such mechanisms from
individual study locations to assess regional and/or global impacts on
oxidising capacity. As a significant fraction of atmospheric processing
occurs in the marine boundary layer, particularly in the tropics, better
constraint of the possible ocean surface source of <span class="inline-formula">HONO</span> is important for a
quantitative understanding of chemical processing of primary trace gases in
the global atmospheric boundary layer and associated impacts upon air
pollution and climate.</p> |
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| ISSN: | 1680-7316 1680-7324 |